The invention is generally related to inspection of component parts and, more particularly, to the ultrasonic inspection of component parts.
Components with complex shaped surfaces are common throughout industrial and government operation. Many times, the precise shape of these surfaces is not known prior to the need to perform an inspection. For example, in the commercial nuclear reactor industry there are many complex curved components and welds. Currently available qualified methods and procedures for performing ultrasonic (UT) examination of pipe welds from the outside surface are applicable only to geometries that are essentially flat and smooth. Field experience gained to date with these procedures shows that a significant portion of these welds are neither flat nor smooth. Typical field conditions encountered by UT examiners include variations in weld crown conditions and a range of other surface irregularities such as diametrical weld shrinkage.
Phased array ultrasonic testing of industrial components has gained widespread use in the past two decades. Phased array ultrasonic testing is a form of ultrasonic testing in which the transducer is comprised of an array of small individual elements, each with their own pulser and receiver channel. Each element is small so as to insure wide beam spread. On transmission, each element is pulsed at a precisely determined time so that the ultrasonic wave from each element arrives at the same time at a focal point in the components volume. On reception, the signal from each element is delayed by a precise time so that the signals reflected from the focal point in the component's volume will all be in phase. The delayed received signals are then summed together, producing a maximum amplitude signal when the reflection originates from the focal point. Since these time delays for transmission and reflection are electronically applied, they can be rapidly changed. This allows the phased array beam characteristics to be programmed and changed rapidly, at rates as high as 20,000 times per second. Using these techniques, the ultrasonic beam's focal point can be electronically swept in angle, swept through a range in depth, swept linearly along parallel to the probe, or any other desired pattern. This versatility has led to wide spread use of phased array ultrasonics in industrial testing.
Components with curved, wavy, or irregular surfaces have long been a challenge for ultrasonic testing. If the surface has a known regular geometry (such as a cylinder), in some cases an ultrasonic probe can be designed and fabricated to allow the component to be inspected. Using phased array ultrasonics, it is possible to compensate (in many cases) for known surface geometries by adjusting the delay times used in transmission and reception. Focal law calculators are commercially available that allow phased array ultrasonic beams to be designed for simple regular surface geometries. However, when the surface profile varies in unknown ways along the surface prior to inspection, no method has been available to perform ultrasonic inspection. This may occur, for example, when a pipe has been welded and then the weld ground to a smooth finish but not flat. Oftentimes a water path is used between the transducer and the part to conduct the ultrasonic waves into and out of the part, making it easier to perform automated scanning and to obtain reliable coupling.
FIGS. 1 and 2 schematically illustrates the effect of a surface irregularity on an ultrasonic pulse. FIG. 1 shows how the ultrasonic beam on a flat surface is readily focused to the desired area. FIG. 2 shows how the ultrasonic beam is dispersed due to the curved surface. Clearly, if the fidelity of the ultrasonic inspection is to be maintained, variations in the surface profile must be accommodated.
U.S. Patent Application, Publication No. US 2005/0150300, entitled “Device and Method for Ultrasonic Inspection Using Profilometry Data” discloses a method of using several ultrasonic probes on a sled arrangement with water coupling between the transducers and the part surface. The arrangement provides the ability to map out the surface profile in order to correct the inspection of the component by compensating for the error in location of reflectors caused by the uneven surface. However, it does not incorporate the ability to correct the aberration in the ultrasonic beam caused by an uneven surface profile over the width of the ultrasonic beam. Thus, its applicability is restricted to surfaces that can be described as flat over the width of the beam as it enters the surface. This restriction is very limiting since, as seen in FIG. 2, even a 1.5 inch radius of curvature causes severe beam distortion on a typical ultrasonic beam.
A flexible phased array probe has been developed that can be conformed to the surface of complex geometry components to perform ultrasonic inspections. This is described in an article entitled “A Flexible Phased Array Transducer for Contact Examination Of Components with Complex Geometry”, presented at the 16th World Conference on Nondestructive Testing in Montreal, Canada Aug. 30-Sep. 3, 2004. In this experimental work, a flexible piezoelectric probe has been developed that has displacement measuring devices attached to each individual element. The individual elements are pushed down to conform to the uneven surface profile. By measuring the vertical position of each individual element with the displacement sensors, phased array delays can be calculated that compensate and eliminate the beam distortion that the irregular surface would otherwise produce. However, maintaining sufficient contact between the probe elements and the part surface to couple the ultrasonics into the part reliably is problematic. To insure good ultrasonic coupling between the elements and the part, a gap of no more than a couple thousandths of an inch filled with a coupling liquid or gel is required. In many practical cases, the surface conditions make this not possible.
An article by C. Holmes, B. Drinkwater, and P. Wilcox, Insight, Vol. 46, No. 11, 677-680 (2004) discloses the use of monolithic (single element) ultrasonic transducers. The ultrasonic beam characteristics are set and cannot be changed. One probe operates with a water path to measure the surface profile and a different probe for the internal inspection is in contact with the surface of the component. Since the ultrasonic probes described in this reference are not flexible, they can only operate on flat surfaces. The flat surface of the probe must be in contact with a flat surface of the component over the entire area of the probe, with a thin liquid couplant between the probe and the component surface not more than a couple of thousandths of an inch thick in order for it to operate properly. Therefore, this device can only work on components that are comprised of flat surface segments. Over the transition regions from one flat area to another, the internal inspection probe cannot operate. This approach can only accommodate curved surfaces that have such a large radius of curvature that they are essentially flat over the area of the probe. For typical transducers, the radius of curvature must be greater than a couple of feet in order to use this approach. The correction provided by this approach is limited to correcting the location of an internal reflection from the ultrasonic data for the variations in the surface profile. Since the beam angle and distance to the reflector from each probe is known and by knowing the exact location of the probe on the surface and the surface profile, the location of the reflector relative to a global coordinate system can be calculated with the approach described in this article. However, the approach is applicable only to a relatively small subset of the uneven surface components that exist. This approach does not address at all the problem of beam distortion that is caused by a surface that is not flat over the area of the probe. This emphasizes that this approach is only applicable to segments of a surface that are flat over the area of the probe.